Brownouts and Blackouts: What’s the Difference?
What is a Brownout?
According to Wikipedia, a brownout is an intentional or unintentional drop in voltage in an electrical power supply system. Intentional brownouts are used for load reduction in an emergency. The reduction lasts for minutes or hours, as opposed to short-term voltage sag (or dip). The term brownout comes from the dimming experienced by lighting when the voltage sags. A voltage reduction may be an effect of disruption of an electrical grid, or may occasionally be imposed in an effort to reduce load and prevent a power outage known as a blackout. Different types of electrical apparatus will react in different ways to a sag. Some devices will be severely affected, while others may not be affected at all.
In most cases, brownouts are deliberately produced by energy providers as an emergency measure to prevent the system from failing completely (blacking out). Typically a utility will decrease system voltage by 10-25%, usually for a short period of time. This reduction typically has minimal effect on heat and lighting systems, most of which can function reliably for short periods on suboptimal voltage, but sensitive electronic equipment requiring reasonably precise voltages may not be able to function and long-term brownouts can cause premature wear in non-electronic devices. Computer disk drives often suffer write failures when supplied with suboptimal voltage, and electric motors tend to run hotter when required to produce the same horsepower during a brownout. Normal fluctuations in voltage do not qualify as brownouts. System voltage in many service areas can vary by as much as five percent above or below “nominal” line voltage. Manufacturers of electrical and electronic products know this. Most North American consumer and commercial products are designed to function normally and safely for long periods at voltages ranging from 115 to 125 volts.
What is a Blackout?
A blackout is a complete interruption of power in a given service area. Rolling blackouts are controlled and usually preplanned interruptions of service. A brownout is a partial, temporary reduction in system voltage or total system capacity. Blackouts come without warning, last for indeterminate periods, and are typically caused by catastrophic equipment failure or severe weather. The nature and cause of the blackout determines who is affected. Rolling blackouts typically occur with at least some advance warning, normally last for a fixed length of time, and are deliberately produced by utility companies. They can be used as a means of coping with peak power demands that cannot be met from existing supply. Rolling blackouts are usually intended to affect only a specific service area, and the energy provider will typically spread these blackouts among several service areas to insure that no specific area suffers substantially more than any other. Planned outages and rolling blackouts differ slightly in that planned outages are usually announced well in advance and are most commonly needed to allow for routine maintenance, while rolling blackouts can occur with relatively little warning and are intended to take stress off of the system’s energy load.
Brownouts and blackouts refer to power outages, with a brownout being a temporary drop in voltage and a blackout being a complete loss of power. Brownouts can cause appliances to run at reduced capacity or flicker, while blackouts can be dangerous, especially for those who rely on electricity for medical equipment or other life-saving devices. It is important for individuals and communities to have plans in place for dealing with power outages, including having backup power sources and knowing how to stay safe during an outage.
Brownouts can cause unexpected behavior in systems with digital control circuits. Reduced voltages can bring control signals below the threshold at which logic circuits can reliably detect which state is being represented. As the voltage returns to normal levels the logic can latch at an incorrect state; even can’t happen states become possible. The seriousness of this effect and whether steps need to be taken by the designer to prevent it depends on the nature of the equipment being controlled; for instance, a brownout may cause a motor to begin running backwards. The under voltage condition that accompanies brownouts and power outages has expensive repercussions, like increased motor failures and lost production. However, quick response techniques and computer-controlled equipment can reduce or even eliminate the effects of such low-voltage events.
Basic safeguards include monitoring the supply voltage and training personnel to respond quickly if it drops to a predetermined level. For critical applications, install thermal protectors or condition-monitoring devices that can detect abnormally high winding temperatures and shut down the motor. If shutting down isn’t an option, reduce the load by throttling back a fan or partially closing a valve during a brownout. If you don’t take proactive steps, you could be setting yourself up for serious problems. Turning a motor on and off that quickly can damage the windings. The effect is a bit like starting a motor at 1½ times rated voltage, so relying on a holding coil to interrupt the motor isn’t the best option. In the old days, an operator made a best-guess reaction to what he thought was happening. Right or wrong, once someone turns off the lights you’re stuck with the consequences of cleaning up the process line and restarting your systems.
Thanks to computers, just about everything can be controlled more accurately now, including electric motors. Devices smaller than a breadbox hold the guts to start the motor, monitor the power quality, and respond the way an experienced operator would — every time. Instead of guessing whether the voltage is low, or unbalanced, the controller knows and handles the situation for you.
If you are considering back up power for your company or home, find out if you will be affected by PG&E’s planned power outages and other wildfire Mitigation Efforts.